Steel laminations are typically annealed in continuous roller hearth furnaces to improve magnetic properties. The known prior art furnace includes heating and cooling sections followed by a purging vestibule and an oxidation section.
Motors, transformers and other electrical devices typically comprise an assembly of precision stamped steel parts called laminations. The steel laminations form the magnetic path which allows transfer of magnetic flux.
The performance of laminations depends on the magnetic properties of the steel used to make the laminations. Typically the best magnetic properties are achieved following heat treatment or annealing of the laminations after stamping.
In the prior art, rows of laminations are typically loaded onto trays for processing through a continuous roller hearth furnace. As a result of the difference and variations in size and shape, special practices and precautions are required with respect to loading to ensure uniformity of heat treatment during the annealing operation.
It is known in the prior art to provide a purge or transfer vestibule to separate atmosphere in the controlled cooling and heating sections of the furnace from ambient conditions. The atmosphere in the cooling section comprises a decarburizing gas mixture subject to ignition or explosions when mixed with air. The function of the purge or transfer vestibule is to provide an inert atmosphere barrier so that laminations may be removed from the potentially explosive gas mixture without contact with air. Operating conditions for the purge section or vestibule usually require that the static pressure is maintained at values slightly higher than the static pressure of the controlled cool section to minimize leaks of potentially explosive gas.
It is also known in the prior art to create or form a blue/gray oxide on the surface of the steel laminations after the cooling section and purge vestibule in the oxidation section which follows. This oxide serves to provide a measure of electrical insulation for each of the laminations. The oxide is known to be formed in the prior art in the controlled oxidation section of the furnace. The composition of the oxide is a complex mixture of Fe2O3 and Fe3O4. Control of uniform appearance or cosmetics of the laminations is important and difficult to achieve. One of the main rate limiting steps in this process is the diffusion of the oxidation species between the faces of the laminations, which are wired together in rows. When controlled oxidation sections are used in the prior art in an annealing furnace, it is normal practice to use a mixing or re-circulating fan so that the atmosphere can circulate and fully react with all surfaces of the steel laminations. It is known in the prior art that optimum temperatures for oxidation of steel to form a blue/gray reaction product range from 600 to 1000° F. Typical resident times exceed twenty minutes since the rate of oxidation, and therefore thickness of the oxide, depends on the rate of diffusion of oxygen through the oxide layer as it is formed.
After reaction to form a controlled oxide on the surface of the steel, the laminations are allowed to cool in air to a temperature that permits manual handling.
In the prior art oxidation section, it is a conventional practice to use paddle fans on the roof of the oxidation section to provide mixing and circulation of the oxidized atmosphere.